TUSTIN, Calif. -- A violent rampage that left four dead in suburban Orange County began in the pre-dawn hours Tuesday when a 20-year-old killed a woman in his home and sped away in his parents' car, authorities said.

An hour later, it was over -- but not before Ali Syed had killed two more people during carjackings, shot up vehicles on a busy freeway interchange and left three others injured in a trail of carnage that stretched across 25 miles.

One driver was forced from his BMW at a stop sign, marched to a curb and shot in the back of the head as other commuters watched in horror.

Syed later killed himself. He lived with his parents at the Ladera Ranch residence where the first victim, an unidentified woman in her 20s, was slain, Tustin police Chief Scott Jordan said. He was unemployed, taking one class at a local community college, Jordan said.

The woman was not related to the shooter and it wasn't known what she was doing at the home, said Orange County sheriff's spokesman Jim Amormino.

Syed's parents were in the house at the time, fled the residence when shots were fired, and reported it, he said.

Jordan said Syed stated to one carjacking victim: "I don't want to hurt you. I killed somebody. Today is my last day."

Jordan said there was no indication of a motive, but he sought to assure residents that the violence was over.

"There is no conspiracy here, there are no outstanding suspects, it was a very, very unfortunate situation, but I don't think the people here in Orange County have to be worried about their safety," he said.

The violence began at 4:45 a.m., when deputies responded to a call from Ladera Ranch, a sleepy inland town about 55 miles southeast of Los Angeles. They found the woman shot multiple times.

Jason Glass, who lives across the street, said he couldn't sleep and was watching TV in his garage with the door partly open when he heard what sounded like gun shots.

Then he heard a commotion and the sound of a car speeding away.

Hours later, his neighborhood was flooded with police, and crime scene tape sectioned off the street.

"I just happened to be in here when this happened," Glass said about his garage. "To think he could have rolled under my door or needed a car or needed to hide is crazy. It's freaking me out."

From Ladera Ranch, police said the gunman headed north and pulled off Interstate 5 in Tustin, about 20 miles away, with a flat tire and other damage to his parents' car.

A man who was waiting in a shopping center parking lot to carpool with his son saw Syed had a gun and tried to escape in his Cadillac, Jordan said. Syed ran after the car as it drove away and fired his shotgun through the back window, striking the driver in head but not killing him.

The driver "noticed that he was loading his shotgun, so he simply gets back in his car and tries to escape," Jordan said. "He's driving through the parking lot trying to get away and the suspect is actually chasing him on foot, taking shots at him."

Syed then crossed the street to a Mobil gas station, where he approached the driver of a pick-up and asked for his keys, Jordan said.

"He says something to the effect of, 'I've killed somebody. Today's my last day. I don't want to hurt you. Give me your keys,"' he said. "He hands over the keys and he gets in the truck and leaves."

Syed got back on the freeway, where he pulled to the side of the road at the busy I-5 and State Route 55 interchange and began firing at commuters, Jordan said.

One driver was struck in the mouth and hands. He didn't have a cellphone, but was able to drive home and call police. Two other cars were hit but their drivers weren't injured, Jordan said.

"All of this is happening so quickly," he said, estimating that Syed shot at drivers from the side of the freeway transition for about a minute.

The shooter then exited the freeway in nearby Santa Ana but ran the curb and got his car stuck, authorities said.

He approached Melvin Edwards, a 69-year-old from Laguna Hills who was on his way to his Santa Ana business, as Edwards sat in his BMW at a stop sign. Syed forced Edwards to get out of his car, marched him across the street and shot him three times, including in the back of the head and the back, as horrified drivers looked on, Jordan said.

"They tried to get away. They saw what was going on, they tried to get away and they called police," he said.

Syed took Edwards' BMW and next popped up at the Micro Center, a Tustin business, where he shot and killed construction worker Jeremy Lewis, 26, of Fullerton. Lewis' co-worker rushed to intervene and was shot in the arm, Jordan said.

Syed took the second construction worker's utility truck and fled to Orange, this time with California Highway Patrol officers in pursuit. He jumped from the moving utility truck at an intersection in Orange, about five miles away, and shot himself immediately, Jordan said.

A shotgun was recovered at the scene.

A message left at Syed's parents' home wasn't immediately returned on Tuesday.